A six- or seven-speed transmission is known from printed publication DE 103 05 241 A1. The double clutch transmission comprises two clutches, each connected with its input side to the input shaft and with its output side to one of the two transmission input shafts respectively. The two transmission input shafts are arranged coaxially to one another. Moreover, two countershafts are arranged axially parallel to the two transmission input shafts, the idler gears of which engage with fixed gears of the transmission input shafts. Moreover, axially displaceable coupling devices are held in a rotationally fixed manner on the countershafts, in order to shift the respective toothed gear wheels. The respectively selected ratio will be transmitted via the output gears to a differential gear. In order to implement the desired ratio step in the double clutch transmission of the prior art, a multitude of gear planes is necessary, such that a significant amount of construction space is needed for the installation.
Moreover, a spur-gear multi-speed transmission is known from printed publication DE 38 22 330 A1. The spur-gear multi-speed transmission comprises a double clutch transmission that is shiftable under load, one part of which is connected with an input shaft and another other part with a rotatable hollow input shaft placed on the input shaft. For certain ratios, the input shaft can be coupled via a shift element with the input hollow shaft.
A powershift transmission is known from publication DE 10 2004 001 961 A1 having two clutches that are allocated each to a subtransmission. The transmission input shafts of the two subtransmissions are arranged coaxially to one another and are meshed via fixed gears with idler gears of the allocated shaft elements. The respective idler gears of the countershaft can be connected in a rotationally fixed manner with the respective countershaft through allocated shift elements. An eight-speed transmission is known from this publication having another shift element to couple the two transmission input shafts in order to obtain another transmission ratio step. In this embodiment, the seven-speed transmission requires at least six gear wheel planes in the subtransmission in order to obtain the transmission ratio steps. This creates an undesirable extension of the construction length in axial direction, thus, considerably limiting the installation possibility in a vehicle.
Moreover another powershift transmission is known from publication DE 10 2005 028 532 A1 comprising two input shafts and only one countershaft. For example, an eight-speed transmission requires in this embodiment more than seven gear planes, in order to obtain the transmission ratio steps, in particular the reverse ratios. This creates an undesirable extension of the installation length in axial direction.